Soldering is widely employed for the fabrication of many different products, especially products in the electronics field, notably printed circuit boards. For the production fabrication of printed circuit boards and similar products, wave solder systems have been widely employed. These systems include a reservoir of molten solder which is pumped in a wave, with the circuit board being transported in contact with the wave to cause wetting of the intended areas of the board. The solder wave serves as a source of heat for heating of the circuit board and also as the solder source for application of solder to the board. The wave solder apparatus is relatively complex and must be carefully designed and constructed to provide a solder wave of the appropriate dimensions and characteristics to provide the requisite heating and solder application to the particular product being processed. The length of the wave must be sufficient to provide sufficient time of contact to heat the surface to be soldered. The wave design is critical in relation to the product and speed of the product through the wave. There is a maximum speed beyond which a product cannot be conveyed through a particular solder wave and still achieve sufficient heating. For greater speeds, the wave would have to be reconfigured, which would necessitate a complete system redesign.
The height of the solder wave is also a critical factor and can be a limiting factor to the size of a product being processed. For example, in a printed circuit board in which leaded components are inserted, the lead lengths cannot be longer than the depth of the solder wave or else a board cannot be accommodated in the particular wave solder apparatus because of interference with the long component leads. Thus, care must be exercised in the assembly of components on a circuit board to assure that the lead lengths are less than the depth of the solder wave for the particular solder apparatus to be employed. Since the solder wave serves both as the source of solder and source of heating, the dynamics of the system become complex in designing a wave solder apparatus for particular purposes. Also, these dynamics make it difficult to alter the characteristics of the solder wave to accommodate different types of products.
Another type of production solder system is the drag solder system in which a reservoir of solder is provided into which a product is dipped and immersed either partially or completely to apply solder to the product. Here again the molten solder serves both as the source of heat for heating the product to a soldering temperature, and as the solder applicator.
The Ammann et al. U.S. Pat. No. Re. 30,399 shows a wave flow soldering apparatus in which the solder wave is provided at the bottom of a vessel containing a heated, saturated vapor. The product is conveyed through the vapor chamber and is heated by immersion in the vapor phase and soldered by passage through the solder wave. Solder is applied in an anaerobic atmosphere provided by the vapor phase but there is little improvement in the control or performance of the soldering operation by location of the solder wave apparatus in the chamber. The criticality and relative complexity of the solder wave remains the same as described above, and the presence of the solder wave within the heated vapor does not alter the critical design considerations of the wave apparatus.
An article entitled "A New Soldering Process", by W.R. George, Brazing & Soldering, No. 5, Autumn 1983, shows a drag soldering system in which the product is initially placed within a heated inert vapor phase prior to immersion into the molten solder bath.
The Sarnacki et al. U.S. Pat. No. 3,825,164 shows a soldering system having a tank containing a pool of solder at the bottom and covered by a liquid fluxing bath, with solder spray apparatus within the fluxing bath. A printed circuit card is inserted vertically into the fluxing bath for fluxing and preheating, and solder is sprayed onto one or both surfaces of the printed circuit card as the card is withdrawn from the tank.